A mass spectrometry (“MS”) system generally includes an ion source, a mass analyzer, and an ion detector (or ion detection system). The ion source provides for ionizing atoms or molecules of a sample (or analyte) of interest. Various ion optics are also part of the MS system to efficiently extract and accelerate ions from the ion source to form an ion beam (or ion stream) which can be efficiently delivered through the mass analyzer to the ion detector. Provided such ions have the same kinetic energies “E” after extraction and acceleration, their velocities “ν” will vary inversely to their corresponding mass-to-charge ratios (also referred to equivalently as “m/z” ratios, or more simply “masses”), with ions of comparatively smaller mass having greater velocities, and ions of comparatively larger mass having lower velocities, as
  v  =                    [                              2            ⁢            E                                (                          m              ⁢                              /                            ⁢              z                        )                          ]                    1        ⁢                  /                ⁢        2              .  In a time-of-flight (“TOF”) MS system, a pulsed ion stream (as a pulse or “packet”) is provided to the mass analyzer, so that the ions traverse a known distance from the ion source to the ion detector, with ions having the greater velocities arriving comparatively earlier in time at the ion detector and ions having the lesser velocities arriving comparatively later in time at the ion detector. Counting ions at the ion detector simultaneously with recording their differing arrival times thus allows for separation of the ions based on their differing masses. A TOF-MS analysis produces a mass spectrum, which is a series of peaks indicative of the relative abundances of detected ions as a function of their arrival times, corresponding to their m/z ratios. Mass spectrometers are commonly used to determine the chemical composition of solid, liquid and gaseous substances by precise measurement of the mass-to-charge ratio of the constituent atomic and molecular ions.
It is a widely acknowledged consensus in the field of mass spectrometry that there is no ideal mass analyzer for all applications. In TOF-MS, the state of the art is represented by two distinct technologies: reflectrons and electrostatic sectors. Each of these two families of mass analyzers has its own strengths and weaknesses.
As an example, FIG. 1 is a block diagram illustrating such a prior art TOF-MS system embodiment having orthogonal acceleration of ions for the mass analyzer, which is popular (although not dominant) in many molecular MS applications. The prior art TOF-MS system 50 may generally include, in series of ion process flow along the drift axis, a pulsed ion source 54 (comprising an ion source 52, ion optics 56 (optionally including one or more ion guides, not separately illustrated), and an ion accelerator 62), a time-of-flight (TOF) mass analyzer 58 having a reflectron 60 (for this example), an ion detector 64, and a computing device 68. Sample molecules or atoms are introduced into the ion source 52, and the ion source 52 produces ions from sample molecules or atoms and transmits the ions to the ion optics 56, which in turn focus the ions as an ion beam (or stream) 66 and transmit the ions to the ion accelerator 62. The ion optics 56 may perform additional ion processing functions such as compressing the ion beam, and/or thermalizing (cooling) the ions, for example. For entry into the mass analyzer 58, the ions are generally injected as a pulse or packet of ions 70 (using ion accelerator 62), orthogonally to the drift direction (in this case) and toward the reflectron 60. The ions are reflected (generally about 180 degrees) by the reflectron 60 and travel to the ion detector 64, having dispersed based on differing flight times (due to their differing mass-to-charge (m/z) ratios). The ion detector 64 generates a signal, based upon arrival times and/or arrival locations, that is then utilized by the computing device 68 to calculate actual times-of-flight from which m/z ratios are correlated, and provide a mass spectrum descriptive of the sample molecules as appreciated by persons skilled in the art.
A significant problem with these various prior art TOF-MS systems, however, is a potentially large variance in the kinetic energies of the ions generated by the ion source 52. When there is a significant range of kinetic energies of the ions comprising the ion beam, ions having the same masses but differing kinetic energies will have different arrival times at the ion detector 64. Instead of having a narrow peak of arrival times in the mass spectrum for a given mass, there will be a significant spread of arrival times, providing a comparatively wide peak with large tails in the mass spectrum, and potentially obscuring and interfering with detection of ions of nearby masses. In fact, the reflectron TOF-MS was developed from the earlier linear TOF-MS design to compensate for broad ion kinetic energy distributions, although the reflectron does not fully resolve this problem. With mass resolving power defined in any TOF-MS as T/ΔT, the time-of-flight “T” from the ion source to the detector, as divided by the width of the mass spectrum at one-half the maximum (“ΔT”), a larger variance of kinetic energies generates a larger ΔT, due to the corresponding spreading of velocities and flight arrival times, decreasing mass resolving power, and also decreasing the signal-to-noise ratio (“SNR”). Excessive ion kinetic energy distributions can also reduce the mass accuracy, defined as the deviation in the calculated mass from the actual mass of a measured ion.
A need remains, therefore, for a TOF-MS apparatus and system which can select and/or control the kinetic energies of the ions comprising the ion beam, to create an ion beam having a selectable and comparatively narrow band of kinetic energies. Such a TOF-MS apparatus and system should also provide for selectable or configurable time-of-flight “T” and TOF focusing in various system embodiments, and may include multiple TOF focuses and tandem operation. Such a TOF-MS apparatus and system should also selectively preserve spatial information in the ion beam at detection, to allow for stigmatic imaging. In addition, such a TOF-MS apparatus and system should be capable of multimode operation, to selectively operate or configure the TOF-MS apparatus and system for these various features and in various combinations.